Squelch detectors are commonly used in communication links, such as high speed serial links, to provide an indication of when data signals are present on a link as distinguished from noise signals and the like. When the data signal is not present, the receiver can be momentarily turned off, thus limiting power consumption. Over time, such off-intervals can result in increased battery life for hand held receivers and other benefits.
In order to operate effectively, squelch detectors are typically configured with a large gain value and a large bandwidth so as to ensure maximum detection probability. The bandwidth of the squelch detector is generally wider than the input bandwidth of the receiver. Such requirements can reduce the efficiency of the overall receiver since power gains achieved by receiver shut-off during intervals where no signal is detected are lost by the need for additional power to support wider bandwidth can gain requirements of the detection stage.
A typical signal configuration in a modern high speed data link involves differential signal pairs that are of opposite polarity and transition to different and opposite levels during data transitions. Such operation is known in the art as differential signaling and is advantageous, inter alia, because of the inherent noise rejecting properties, tolerance to DC offset and the like. A receiver using differential signaling can experience enhanced common mode noise rejection, although such rejection can be due to the balanced nature of the typical transmission line. The signals on each line are compared only to each other and thus external influences of factors such as DC offset can be minimized.
In order to create a detection envelope, squelch detectors introduce offset as will be described in greater detail hereinafter in connection with, for example, the conventional circuit shown in FIG. 1. By using, for example, a replica circuit, a current mismatch can be introduced on the two branches of a differential amplifier. Signals that fall outside the detection envelope established by the offset are considered data signals while signal energy within the offset is considered noise.
Disadvantages can arise in conventional squelch detection circuits in that that linearity constraint can be exceeded and circuit sensitivities to current and replica matching can arise. Further, the common mode of the replica circuit is fixed, while that of the actual amplifier varies, for example in accordance with universal serial bus (USB) standard version 2.0. Accordingly, a systematic mismatch and hence additional and often non-linear offset can be introduced in a scheme that uses a replica circuit. Consequently, errors can arise due to mismatch and also due to any offset inherent in the replica circuit. Such errors are difficult to compensate for since they may vary across different signal levels and also across different fabrication processes and scales.